Switching system



May 5, 1936. J. G. FERGUSON SWITCHING SYSTEM Filed July 20, 1935 8 Sheets-Sheet 1 FIG. I

IN VE N TOR B J; G. FERGUSON ATTORNEY F4 6. 6 FIG;

FIG 9 5. 6% Fia y 193$ J. G. FERGUSON 2,3@,64

SWITCHING SYSTEM Filed July 20, 1935 8 sheets-sheet- 2 lNVE/VTOR J. G FERGUSON A T TOR/VEV May 5, 136. J. G. FERGUSON I 2,@39,640

.SWITCHING SYSTEM Filed July 20, 1935 8 Sheets-Sheet 5 TRUNKS DIG/T SW FINAL- SWITCH DIGIT.

SWITCH FIG. .3

lNl/ENTOR J. G. FERGUSON A TTORNEY May 5, 193% J. G. FERGUSON SWITCHING SYSTEM 8 Sheets-Sheet 4 Filed July 20, 1935 lN EN TO By J. FERGUSON (r n a" A FM ATTORNE' ay 5, 1936. .1. G. FERGUSON 2,639,640

SWITCHING SYSTEM Filed July 20, 1935 8 Sheets-Sheet 5 FIG. 5

l l/E N TOR J; a. FERGUSON ELM/WW A 7' TORNEV May 5, 1936. J. G. FERGUSON 2,039,640

SWITCHING SYSTEM Filed July 20, 1935 8 Sheets-Sheet 6 INVENTOR J. G. FERGUSON A 7' TORNEV 37 1936.. J. G. FERGUSON Z Q SWITCHING SYSTEM Filed July 20, 1955 8 Sheets-Sheet 7 //v VEN TOR J. G. FERGUSON 9 K. in E A T TORNE V 8 Sheets-Sheet 8 May 5, 1936. J. G. FERGUSON SWITCHING SYSTEM Filed July 20, 1935 INVENTOR Y J. G FERGUSON ATTORNEY Patented May 5, 1936 UNITED STATES PATENT OFFICE SWITCHING SYSTEM Application July 20, 1935, Serial No. 32,318

4 Claims.

This invention relates to switching arrangements for telephone systems and particularly to those employing switches of the cross-bar type.

The object of the invention is to reduce the amount of switching equipment necessary to han dle the calls in a group of lines by giving all lines access to a limited number of regular switches and also access to a number of second-choice or overflow switches.

Heretofore systems have been devised for directing the traflic originating in a group of lines through first-choice switches if idle and through reserve or overflow switches if all of the firstchoice switches are busy. The present invention contemplates the application of this principle to systems utilizing switches of the cross-bar type and the effecting of considerable economies by a novel arrangement of the lines and trunks in these switches.

According to a feature of the invention the lines of a group appear in a limited number of regular cross-bar switches and in a limited number of second-choice or overflow cross-bar switches. The several subgroups of the group appear in respective rows of contacts of each switch, each line having a single appearance in each switch, and each subgroup appearing in non-corresponding rows of contacts in the regular switches and in corresponding rows of contacts in the overflow switches. The outgoing trunks from the regular switches are multipled to all of these switches so that each trunk occupies a corresponding row of contacts in each of said switches. Each of the overflow switches has access to a single outgoing trunk, these trunks appearing in all of the contact sets of the respective overflow switches.

By this arrangement a particular line, instead of occupying all or a substantial part of the contacts in a row as is usual in cross-bar switches of the prior art, occupies a single set of contacts in a row of each of several switches, thus giving it access to a different trunk outgoing from each such switch and also permitting a large number of lines to be served by the switches.

The foregoing and other features of the invention will be more clearly understood from the following detailed description taken in connection with the accompanying drawings, and also from the appended claims.

In the accompanying drawings in which Fig. 1 shows the general plan of the system, Figs. 2 to 8, inclusive, when placed together as indicated in Fig. 9, illustrate the details of one method of extending incoming calls to trunks which lead to first, second, and final digit switches.

These drawings show complete circuit connections between typical calling subscribers lines SH] and S9! and a called subscribers line B. The connection is completed through the medium of any one of the five cross-bar switches C, D, E, 5 F, or G to a trunk leading to a first digit selector switch H, a second digit selector switch J and final switch indicated at K, all of any well known type, to the called subscriber B. The cross-bar switches shown in Figs. 1 to 7 may be of any well- 10 known type, such as the one shown and described in detail in the application of J. N. Reynolds, Serial No. 702,453, filed December 15, 1933, issued as Patent 2,021,329, Nov. 19, 1935.

In the switching arrangement shown in the 15 drawings only one switch contact is shown at each cross-point in the sleeve or control lead of each line, whereas the switch in actual practice has three or more contacts at each cross-point, two contacts being required for the talking leads besides the control lead. The arrangement of switches is such that any contact set is capable of operation upon conjoint displacement in definite order of the two bars, not shown, which intersect in the locality of this set of contacts. The contact sets are maintained actuated by the displacement of the last actuated one of the two bars, and are restored to normal on the release of the last actuated bar. The first bar to be actuated in operating any given contact set is referred to as the horizontal bar and is actuated by a selecting magnet shown immediately to the right of each row of contact sets in each switch, and the second bar to be actuated is referred to as the vertical bar and is actuated by a holding magnet shown directly below the vertical row of contact sets in each switch unit.

The trunk arrangement is shown in schematic form in Fig. 1 wherein one hundred incoming subscriber lines are divided into subgroups of ten lines with provision for assigning an idle trunk to one calling line at a time. Each subgroup has first-choice access indicated at GRl to one of ten trunks, second-choice access indicated at GRZ to a second one of the trunks, third-choice access indicated at GR3 to a third one of the trunks. If all three trunks are busy, then the same subgroup has access, indicated at GR4, to an overflow trunk, and if said trunk is busy the group it has access, indicated at G35, to another overflow trunk. The illustrated system includes five cross-bar switches arranged as primary switches with ten trunks to the digit switches running vertically through the first three switches and two additional overflow trunks connected one to each of the two overflow switches shown at the bottom of Fig. 1. Arranging the line multiple in this manner provides a high degree of trunk accessibility considering the number of line appearances. If the calling rate of these lines requires it, additional switches may be added and the group slip of ten lines extended to one or more trunks before connecting with one or more overflow switches.

Figs. '2 to 5 show three regular cross-bar switches C, D and E wherein each contact provides a means for connecting an individual line with a given trunk, each line having a total-ofjthree TK relay and thereby maintains the leads to the holding magnets always connected to those magnets that are associated with an'idle trunk.

Figs. 6 and '7 show two cross-bar switches'F and G wherein *each 'contactprovides means of .con-

ne'cting an individual'line with one of two overflow trunks also leading 'to first digit selector switches. The :two relays tothe right of these 'switchesin Fig. 7 are usedas busyrelays to transfer the holding magnet leads whenever theassociated trunkis busy. :Fig. .8 shows typicaliline andcut-off relays in which'the subscriber ilines terminate, arranged to representa group of one hundred lines. The first vertical :rowof relays represent lines one to ten, :the second "vertical row representslines eleven :to twenty and a-thir'd -row,'lines ninety-one 'to-one hundred with only 'the multiple:wiring indicated forthe'intermediate Jrelays. Also, a group of control relays such as Al, Bl andDl areshown'at the'extreme'right'of each of the :ten horizontal .rows of line relays (L I 'etc.). These relays in conjunction with. another group 1of:'contr01 relays suchas CI, etc., at the bottom 01 563611 vertical row of line relays, identify zthe-number of the line thatis calling sothat the trunk to which .the line has access may betested for busy as'willzlater be described. The control relays referred to above are further controlled or directed :by relayslocated in .the lower right corner of .Fig. 8 which function to :prevent more than one line at-a time from being connected :to 'an idle trunk as well as :to .direct' the'selection -ofthe idletrunk. A detailed description willnow be given of the. manner in which anincoming "call"may directly 'realchany -one of three trunks that-may be idle, or :if' these are .busy, it will be shown how either one of 'two overflow trunks may be selected. 7

When'the subscriber at station SH], for ex- :ample, removes hisreceiver from the switch hook, *a bridge'is .placedzacross line conductors 2M and 202 of T Fig. 2 which causes line relay Llfl of Fig. .8 to-operate due to ground being connected to :lead 20! at outside right break contact of cutoff 'relay C0! 0 and battery connected through winding .-'relay LlO' to lead 292 at inside break *contact relay CO). The sleeve or control lead, "localto the ofiic'esfor this line, is traced over lead 8! to battery through the winding of relay -C0l0. Upon operating, relayLlfl at its right zinnercontactprepares' azpath forioperating relay @C-l byconnectinglead 802 to lead80'3 as later will Thus, whenever a be described. Relay Llll also closes the circuit for operating relay All) over a path traced from ground on lead 804, right outer contact relay LIB, lead 895, top break contact relay Dl0 to battery through winding relay AID. Relay All) at its front contact 805 connects lead 801 to left Winding relay BIB. Lead 801 being a test lead running through the armatures and .back contacts on other relays Al to A9 will be open at the left back contact relay 808 if another call is already in the process of selecting a trunk. Assuming, however, that relay 808 is not operated, then ground from the break contacts of relays 8% and am will be connected through break contact relay 808, lead 801, contact 806 on relay Alfl' and through left winding relay BIB to battery. Relay BIO operates to close a circuit from battery through the right winding relay Blil andfront contact to another test lead M3 to ground through winding relay 808. Relay 808 operating, then opens lead 801 as mentioned above and prevents later calls .from interfering with the handling of thercall on line H]. Hadit been assumed that calling :isubscribers SI .and

S2 originated calls simultaneously on lines I and 2, then relays Aland .A2 wonldboth have operated but due to test leadfllllonlyrelayBZ would operate and lock to test lead 8I3' thusgiving .the call on line 2 preference over the callon line I. Test leads 801. and 8| 3, therefore, control the .vertical preference and permit the highest :of the calling lines in 'any group of 'tento search for a trunk and lock out all other lines until :an idle trunk is found or until the callis timed out'when another attempt is made toselect a trunk.

Just as the vertical preferencetdescribed above is determined by the group relays: Bl .to B10 and common relay 808 so the horizontal preference permits the highest of the ten subgroups of 'lines to search foratrunk and lock out all other lines. 4

This is determined by the common relay 8| I :and reiays'CI to CH1, inclusive, together with lead 8. Therefore, immediately after the vertical preference is determined, the BIO relay closes a'circuit for'operating relay C! over a path traced'from groundat the top break contact relays '809and 8 I0 through armature and right back contact re- "lay 8| I, thence over multiplelead'8l5; contact 8 f6 7 on relay 'BIO, multiple lead 802, through operated inner contact on relay 'Llfl, multiple lead 803, and through bottom winding relay 'CI to battery. Relay CI, in operating, locks to ground through its top winding and front contact to lead 8M, traced through all the top break contacts/and armatures of relays C2 to CH1, thence-through winding of relay8l I to ground at break contacts of relays 809 and 8|0. Relay 8, in operating,

opens its right break contactwhich breaks-the operating circuit previously traced 'for the lower Winding of relay Cl, thus preventing later calls that might originate in other groups of lines from interfering with thehandling of the call on'line ID. The identity or location of the-calling line is thus established by virtue of the fact that only one of the B and one of the C relays; namely BIO and Cl, remain operated.

Another example illustrating this horizontal preference is given by assuming that subscribers on lines I and 9| lift their'receivers about the same time, in which case line relay LI and corresponding BI and Cl relays operate in a similar manner to that described 'for line H). Line relay L9! also operates and connects ground that is on lead8l 1 tolead 8l8. Since both LI and L! relays connect ground to'lead '818, which path-is further traced through relay Al to battery, it will be evident that only the BI relay will operate. The vertical preference is thus determined by the BI relay being operated, both lines being the topmost one in their respective subgroups. When, therefore, ground is placed on lead 8ID at contact 820 by relay BI, a circuit is established for operating both the CI and CID relays, traced over lead 8I9 to right inner contact relay L9I and lead 82I to battery through lower winding relay CID; also traced over lead 8I9 through inner contact relay LI, lead 803 to battery through lower winding relay CI. In this case the locking circuit through the upper winding relay CID is completed back over lead 8I4 through the winding of relay 8 to ground, while the locking circuit through the upper winding of relay CI is held open at contact 822 on relay CID. Relay 8 operating thus opens at its top break contact the circuit for energizing the lower windings of relays CI and CID so that relay CI releases and relay CID remains operated. In this manner, line 9| is identified as the calling line to be handled first, and connection to an idle outgoing trunk will be directed by the combination of the BI and CID relays remaining operated as will later be described.

Going back to the condition ensuing forthwith upon the identity of calling line I being determined by relay BIO and relay CI remaining in an operated condition, there will now be described the manner in which line ID is connected to an idle trunk. Relays 808 and 8 of Fig. 8 it will be remembered, are also operated at this time which prevents interference from other lines that might originate calls at this time. Since each calling line has access to five different incoming trunks, it will be assumed that the firstchoice trunk 30I for line I0 is idle. Relay TKI of Fig. 3, therefore, is normal because if the trunk were busy this relay would be operated as will presently appear. The circuits of this invention are organized for positive operation by preventing the circuit for a holding magnet from being closed until the selecting magnets have operated. In the case of line ID selecting magnet 3I3 of Fig. 3 will be operated over a path traced from ground at the back contact of relays 809 and 8ID to armature and front contact on relay 808 to multiple lead 823, through contact 824 on relay BID to multiple lead 825, thence through all five selecting magnets in multiple located at the bottom of each cross-bar switch including selecting magnet 3I3 to battery at the bottom of cross-bar switch C. Selecting magnet 3I3,

- when operated, connects ground to lead 3I4 through winding relay DI 0 to battery. Relay DI 0 operating at its top break contact opens the circuit of slow releasing relay AID causing it to restore to normal. At contact 826 relay DID prepares a locking circuit for itself traced to multiple lead 821 to ground at left outer contact relays AIO to AI as will later be explained. At contact 828, relay DI 0 closes a temporary path from ground at contact 829 on relay AID to lead 830, to battery through winding of relay 8I2. Relay 8I2 operates in this circuit and closes a locking circuit for itself through its own contact 83I to ground at back contact on relays 809 and 8ID. Relay 8I2 at contact 832 connects ground to left front contact relay BII, thence to lead 833, for operating the holding magnet 203, Fig. 2, associated with line ID and trunk 30L The holding magnet thus operates in a path traced further through contact 834 on relay CI, lead 835, armature 3I5 and back contact on relay TKl, lead 3I6 to battery through holding magnet 203. The closure of ground to lead 833 also operates slow release relay 809 which in turn operates slow operate relay 8| 0. The latter relay provides a time interval in which to insure operating the holding magnet before ground is removed from the control relays 808, 8 and 8I2. Relay 809 provides another time interval before restoring ground to the control relays, preparing the system to receive another call. The holding magnet in the circuit traced above extends the line ID to the trunk 3DI through cross-bar switch contacts 204.

In the system of this invention upon connecting with an idle trunk, ground is returned over the sleeve or control lead for holding the connecting switches until the subscriber restores the receiver to the switch-hook all in a manner well understood in the art. Therefore, soon after contact 204 in the switch C of Fig. 2 connects the calling line ID with the incoming trunk 30I, ground may be traced on the trunk sleeve 38! through armature and front contact on holding magnet 2G3 to battery, thus maintaining the latter operated until the subscriber hangs up the receiver. At this time, cut-off relay CO-ID of Fig. 8 also operates from ground on trunk sleeve 30I traced through switch contact 204 and lead 205 to multiple sleeve lead DDI to battery through winding relay COID. Relay COID operating releases relay LID, this action taking place before ground is removed by the control relays 808 and 800. Relay COID remains operated as long as the connection is held and prevents relay LID from reoperating. Ground returned on trunk sleeve 30! may also be traced over lead 3I'I, through top break contact relay TKI to battery through winding relay 3I8. The latter relay, when it operates, prepares a path for operating relay TKI so that when relay 8ID operates, ground placed on multiple lead 836 may further be traced through operated contact on relay 3I8 to battery through lower winding relay TKI. The latter relay operates to close a locking circuit through its top winding to lead 3I'I and thence to ground on the trunk sleeve 30 I In this manner, the ground that serves to operate the holding magnet 203 is prevented from operating a second holding magnet which would be the case if relay TKI were allowed to operate upon ground first being connected to trunk sleeve 30I. The operation of relay 8ID also releases relay 808 and selecting magnet 3I3 and in turn relay BID, relay DID and relay 8I2. Relay 809 releases after an interval, thus restoring ground to the control relays for functioning on the next call.

Having described the call from subscriber SID on the assumption that trunk 3Iil was idle, the call will now be. traced on the assumption that trunk 30I is busy and that the next trunk 3I0 is available. In this case it will be observed that the second-choice appearance of line ID is in the cross-bar switch D in the top right-hand portion of Fig. 5. The removal of the subscribers receiver causes the BID relay and the CI relay of Fig. 8 to operate and mark the location of the calling line as previously described. These relays together with relays 808 and 8| I remain operated until the line is connected to the trunk. When relay 808 first operates, ground is connected to lead 823 for operating selecting magnet DI over a path traced through contact 824 on relay BID, lead 825, thence to battery through all five selecting magnets in multiple including selecting magnet 5DI of the switch D. Operating selecting magnet SDI at its lower contact connects ground to multiple lead 3H for operating 012 and connects-ground to-multiple lead v 833 for .operating holding magnet 502.

with trunk 3 l 0,

Ground onlead 833 is traced further through contact 834 on relay CI to lead 835 and thence to the armature Iof relay TKI, but if the associated trunk .30I is busy, as is now assumed, ground on the sleeve lead 30| and lead 3|! holds relay TKI operated and lead 835 is thereby extended over lead 3l9 toarmature 503 of thenext relay TKI 0 associated Relay TK|0 is not operated at this time because trunk 3l0 is idle. Therefore,

ground that is on leads 835 and CH9 is extended over lead 504 for operating holding magnet 502.

The operation by the holding magnet of switch contact 505 thus completes the connection of this line to incoming trunk 310. According to the usual operation ground is returned on the incoming trunk sleeve lead 3| 0, through left-hand looking contact and winding of holding magnet 502 to battery before ground is removed from lead 504 by the slow operate control relay 8I0 of Fig. 8.

When relay 8l0 does operate, it connects ground to lead 836 for'operating relay TK|0 through the operated contact of relay 506 and as previously described prepares the system for the next call.

Letit now be assumed that both trunks 30| and 3| 0 are busy and that the third-choice trunk 309 is available when subscriber Slil removes his receiver from the switch-hook. So far as the selecting magnet circuit is concerned, it may be traced as previously described, because it will be observed that while the selecting magnets on all five switches operate, only one of the holding magnets is caused to operate, thus selecting the idle trunk. Selecting magnet-501 may, therefore, be assumed to have operated and connected ground to relay 'DIO for operating relay 8l2 which connects ground'through contact on relay 8 to lead 833. The holding magnet 508 of the cross-barswitch E in Fig. 5, therefore, is operated from ground on lead 833 through contact 834 on relay CI, lead 835, armature 3| and front contact on relay TKI, lead 3l9, armature 503 and front contact on relay TKIO, lead 509 through armature 5l0 and back contact on relay TK9, lead 5 to battery through holding magnet 508. The operation of the holding magnet-at'switch contact 5l2 thus completes the connection of line l0 to incoming trunk 309 and according to the usual operation,ground is returned by the incoming trunk on sleeve lead 309, through left locking contact and winding of holding magnet 508 before ground is removed from lead 5 by theslow operate control relay'B I 0 of Fig. 8. When relay BIO finally operates and connects ground to lead B36,the relay TK9 will be operated through the contact on relay 5 l3 and the system prepared for the next call as previously described,

,If incoming trunks 30l, 3l0 and 3! are busy when subscriber Sl0 originates a call, then one of the overflow trunks OFI or OF2 will be preempted to connect the subscriber with a first digit switch, such as H. For a description of this condition it will be assumed that incoming trunk OFI is idle. As previously described, the lifting of the subscribers receiver results in-the operation of all five selecting magnets connected to lead 825 which includes selecting magnet '10 of the cross-bar switch F. Selecting magnet IGI at auxiliary contact 102 connects ground to lead 3I4 for operating relay Dl0. Relay Dl0 operates relay 8I2 which connects ground through contact on relay 8 to gleadi833. ,The holding magnet 60] of switch E is e p r ed "ove h am :path ;;!:r0m r und n* a 3,3 as .d ihedi h prec i paragraph, except, at armature 5| 0 on relay TK9 the circuit is traced further through; the front operated contact over lead ,5IA, armature I03 and 601 to battery for holding theswitch-contact602 operated during the timetheline is busy. This ground also may be traced through contact 602 g to lead 603 and thence over-multiple lead 80l vto battery through 'relay'COI0 of Fig. 8 for preventing further operation of relay LI 0. The same ground is further traced through contact 105 on 'relay TKH to battery through winding of relay;

7,06. Relay 106 operating, prepares a 'path for operation of relay TKII when slow relay 8I0 finally operates to connect ground to lead 836.

The operation ofrelay TKll'thus transfers the operating circuit for the holding magnets of cross-bar switch F to the magnets oficross-bar switch G ready for themext call.

The description given in the preceding paragraph assumed overflow trunk OFI to be idle.

If this trunk-were also busy, then the-callon line .10 would be connected .to overflow trunk 0E2, assuming it to be idle. The tenth selecting magnet onall fiveswitchestwilloperate from multiple lead 825, asbefore stated,,including selecting magnet 101, thereby connecting ground;

to lead 3| 4 for operating relayDl-D. Relay-DH operates relay 8 l 2 which connects ground through contacton re1ay.8l-l to lead.833. The holding magnet 6040f switch G is thus operated over the same path from ground onlead ,833 as described connection of .line I0 to incoming trunk OF2. .Groundreturned on sleeve lead QF2 of incoming .trunk, therefore,-may now be traced throughthe armature andfrontcontaction holding magnet .604 to batteryfor holdingthe switch contact605 operated during the time the line is-busy. This groundalso may be traced throughcontact 605 to lead ,606 and thenceover multiple lead 8.0l to battery through cut-off relay COI0. Thesame ground ,is also traced through contact HI on relay TKIZ to batterythrough winding of relay H2. operating relay TKl2 upon the closureofground to lead 836.by.relay,8l,0. Theloperation of relay 'IKIZ thus opens the ,circuitfor all holding ,magnetsand prevents anyfurther trunk selection from taking place.

If subscriber S10 shouldoriginate a call when trunks 30!, 3l0, 3,09, trunks 0F! and OF2, are all busy, then the five selecting magnetsoperate as before,,but, relay TKIZ being operated, the holding magnetcircuit is held openuntil relays809 and M0 operate when anotherattemptto select y a trunk will be made.

When simultaneousv callsare originated by. subscriber stations ,SI and SN, the call on line 9| will be handled first as previouslydescribed. In

this case, relaysB I and-1C L0; remaininan operated Relay H2 operatedgcloses the path .for

position and for the purpose of this description it will be assumed that the first three trunks to which this line has access, namely, trunks 3"), 309 and 308 are busy. Also, that the first overflow trunk CF! is idle. Trunk busy relays TKIO, TK9 and TK8 are, therefore, operated and relay TKII is normal when control relay 808 first operates to close ground to lead 823. This ground path is further traced through contact 831 on relay Bl to multiple lead 838 and thence to battery through the first selecting magnet winding on each of the cross-bar switches C to G, including selecting magnet H3 of switch F in Fig. '7. The latter magnet, upon operating, connects ground at its auxiliarycontact l l 4 to multiple lead 1 l5 for operating relay DI which then looks through its own contact to multiple test lead 821. If one or more calls are waiting in other trunk groups for a trunk selection, their corresponding A2, etc., relays will be operated and will be connecting ground to the test lead 821. This prevents the release of relay DI after a trunk is assigned to the calling line and thereby prevents other calls in the subgroup of lines from receiving trunk assignments until one call in each of the other subgroups of lines have received an assignment. This, in efiect, prevents calls in a particular group looking out calls in less favored groups. Referring again to the operation of relay Dl it will be observed that the circuit to relay Al is opened at the top break contact relay DI. Relay AI is, however, sufficiently slow in releasing to permit ground at contact 839 to operate relay 8| 2 over lead 830 and contact 840 on relay DI. The operation of relay 8l2 connects ground at contact 832 to lead 833 through left contact and armature on relay 8| I. Ground on lead 833 through bottom contact relay CH1, lead 8 causes the holding magnet H6 to operate. Because of the trunks 310, 399 and 308 being busy, this path over lead 84! is further traced through armature 5l5and front contact relay TKIO, armature 5l6 and front contact relay TK9, armature 5| 1 and front contact relay TKB, lead M8 to armature I I1 and back contact relay 'I'Kl l, to battery through winding holding magnet H6. The operation of the latter magnet thus connects line 9| with incoming overflow trunk OFI at contact H8 in the bank of the cross-bar switch F. The system is then prepared for another call by releasing selecting magnet H3, operating relays 809 and 8H] and restoring other control relays normal, all as previously described.

From the above description it will be apparent that this system provides a flexible trunking arrangement whereby subscriber lines in a crossbar system may be given almost any number of accesses, depending on the amount of traffic required by said line.

What is claimed is:

1. In combination, switches, each having rows of separately operable contact sets, a group of incoming lines appearing in non-corresponding rows of contacts in a portion of said switches as first-choice switches and in one of said switches as an overflow switch, outgoing lines appearing in said first-choice switches, each outgoing line appearing in corresponding rows of said first-choice switches, and an outgoing line appearing in said overflow switch.

2. In combination, switches, each having coordinate rows of separately operable contact sets, a group of incoming lines appearing in non-corresponding rows of contacts in a number of said switches assigned to said lines as regular switches and in one of said switches assigned to said lines as an overflow switch, each line having only one appearance in each of said switches, outgoing lines appearing in said regular switches, each outgoing line appearing in corresponding rows of said regular switches, and an outgoing line appearing in a l of the rows of said overflow switch.

3. In combination, cross-bar switches, each having a plurality of coordinate rows of contact sets, subscribers lines arranged in a plurality of groups, certain said switches serving said lines as regular-choice switches and other of said switches serving as overflow switches, said groups of lines appearing in non-corresponding vertical rows of contacts in each of said regular switches and in corresponding vertical rows of contacts in said overflow switches, outgoing trunks multipled to each of said regular switches, and an outgoing trunk for each of said overflow switches.

4. The combination in a telephone system of cross-bar switches, each having a plurality of coordinate rows of contact sets, lines arranged in a plurality of groups, said switches serving said lines in a successive preferential order, each group of lines appearing in certain of said switches in noncorresponding rows of contacts and in others of said switches in corresponding rows of contacts, and trunks accessible to said lines through said contacts, some of said trunks being multipled to a number of said switches and others of said trunks appearing in a single one of the switches.

JAMES G. FERGUSON. 

